1. Field of the Invention
The invention relates to a shadow mask to be used for a cathode ray tube, having a plurality of through-holes, such as dot holes and slot holes, each of which is defined by a greater-size recess formed at a first surface thereof and a smaller-size recess formed at a second surface thereof. The invention further relates to a method of fabricating the shadow mask, and still further to a cathode ray tube including the shadow mask.
2. Description of the Related Art
One of conventional color cathode ray tubes has been suggested in Japanese Unexamined Patent Publication No. 7-65738. FIG. 1 illustrates the suggested color cathode ray tube. The illustrated color cathode ray tube 11 includes a bulb 12 having a face panel 13 constituting a front surface of the bulb 12, and a neck portion 12a, a fluorescent film 14 formed on an inner surface of the face panel 13, a shadow mask 15 disposed in facing relation with the fluorescent film 14 and having a plurality of slots, an electron gun 16 disposed in the neck portion 12a of the bulb 12, and a deflecting yoke 18 disposed around the neck portion 12a of the bulb 12 for deflecting electron beams 7 emitted from the electron gun 16.
In operation, the electron gun 16 emits the electron beam 7, which is deflected by a magnetic field generated by the deflecting yoke 18. The deflected electron beam 7 passes through the shadow mask 15, and scans the fluorescent film 14 therewith. In accordance with the scanning path, a certain image is produced on the fluorescent film 14.
In order to enhance basic characteristics expected in an image display device, such as contrast and brightness, the color cathode ray tube is designed to include, on an inner surface of the pace panel 13, a black matrix film (not illustrated) comprising non-luminous light-absorbing material, such as black carbon, filling spaces formed between red, green and blue fluorescent luminous pixels, and a metal back film (not illustrated) which is made of an aluminum film and which reflects light independently of the fluorescent film 14. The above-mentioned fluorescent film 14 is integrally formed with the black matrix film. The shadow mask 15 is disposed in facing relation with the metal back film.
Hereinbelow is explained the shadow mask 15 having a plurality of rectangular slots through which the electron beam 7 passes.
As illustrated in FIG. 2, the shadow mask 15 is formed with a plurality of slots 22 each of which has a longer side in a direction of a vertical axis V and a shorter side in a direction of a horizontal axis H. Bridge portions 23 are formed between the adjacent slots 22 in the vertical axis V direction, and connecting portions 24 are formed between the adjacent slots 22 in the horizontal axis H direction.
Each of the slots 22 is a through-hole comprised of a first recess 25 formed at a first surface of the shadow mask 15, and a second recess 26 formed at a second surface (not seen in FIG. 2) of the shadow mask 15 and having a smaller size than the first recess 25. Herein, the first surface of the shadow mask 15 is defined as a surface facing the fluorescent film 14, and the second surface is defined as a surface facing the electron gun 16. The slots 22 are formed by the steps of forming a first photoresist pattern on a first surface of a thin metal plate for forming the first recess 25, which first photoresist pattern defines a plurality of rectangles each of which has a longer side in the vertical axis V direction and a shorter side in the horizontal axis H direction, forming a second photoresist pattern on a second surface of the thin metal plate for forming the second recess 26, which second photoresist pattern also defines a plurality of rectangles each of which has a longer side in the vertical axis V direction and a shorter side in the horizontal axis H direction where the longer and shorter sides in the second photoresist pattern are shorter than those in the first photoresist pattern, etching the thin metal plate with the first and second photoresist patterns acting as a mask to thereby form the first and second recesses 25 and 26, and removing the first and second photoresist patterns.
FIG. 3 is a cross-sectional view taken along the line IIIxe2x80x94III in FIG. 2, illustrating a positional relation between the slot 22 and the incident electron beam 7 passing through the slot 22. As illustrated in FIG. 3, if the electron beam 7 partially strikes an inner surface 26a of the second recess 26, a part of the electron beam 7 is randomly reflected in a direction different from a direction in which the electron beam 7 is originally directed. If the randomly reflected electron beam 7a was directed towards the fluorescent film 14, an undesired image would be generated on the fluorescent film 14 by the randomly reflected electron beam 7a, which is a major factor for degrading the contrast of the shadow mask 15.
The electron beam 7 enters, at a greater incident angle, the slot 22 located farther away from a center of the shadow mask 15, and accordingly, is reflected at the inner surface 26a of the second recess 26 to greater degree, resulting in that the contrast of the shadow mask 15 is considerably degraded.
In view of the above-mentioned problem of the conventional shadow mask, it is an object of the present invention to provide a shadow mask capable of reducing electron beams reflected from an inner surface of a through-hole towards a fluorescent film to thereby prevent images from being unnecessarily formed on the fluorescent film. It is also an object of the present invention to provide a method of fabricating the shadow mask, and a cathode ray tube including the shadow mask.
In one aspect of the present invention, there is provided a shadow mask to be used for a cathode ray tube, defining a first region where a plurality of through-holes through which electron beams pass are formed, and a second region where no through-holes are formed. Each of the through-holes is defined by a first recess formed at a first surface of the shadow mask and a second recess formed at a second surface of the shadow mask, and has a first wall farther away from a center of the shadow mask than a second wall thereof. The second recess has a smaller size than that of the first recess. The first wall is formed of a first wall portion defined by an inner surface of the first recess and a second wall portion defined by an inner surface of the second recess. Through-holes located at a marginal region of the first region are designed to have the second wall portion designed to reduce electron beams reflected therefrom in directions different from a direction in which the electron beams are originally directed before the electron beams enter the shadow mask.
For instance, the second wall portion of the through-holes located at a marginal region of the first region may be designed to have such a configuration that electron beams reflected therefrom are directed to an inner surface of the first recess. It is preferable that the inner surface of the first recess is designed to have such a configuration that the electron beams directed thereto are reflected therefrom in a direction in which the electron beams are originally directed.
It is preferable that a first boundary between the first and second recesses within the first wall is located lower than a second boundary between the first and second recesses within the second wall on the basis of a bottom of the second recess. It is preferable that the first boundary has a height equal to or lower than 20 xcexcm on the basis of a bottom of the second recess.
The second wall portion may be designed to have a configuration defined as a function of a horizontal distance between (a) a first boundary between the first and second recesses within the first wall and (b) an outer edge of the second recess, the horizontal distance being defined as a function of a thickness of the shadow mask, a height of the first boundary, a width of the through-hole, an incident angle of the electron beams at the first boundary, and an inner width of the first recess. For instance, the above-mentioned horizontal distance is defined by the following equation:
S3xe2x89xa7H2xc3x97tan xcex21
xcex21=(90xe2x88x92xcex1xe2x88x92tanxe2x88x921((Txe2x88x92H2/(A+S4)))/2
wherein: S3 indicates the horizontal distance; H2 indicates a height of the first boundary; xcex1 indicates an incident angle of the electron beams entering the through-holes,; T indicates a thickness of the shadow mask; A indicates a width of the through-holes; and S4 indicates a horizontal distance between (a) a boundary between the first and second recesses within the second wall and (b) an outer edge of the first recess.
As an alternative, the second wall portion of the through-holes located at a marginal region of the first region may be designed to have such a configuration that electron beams reflected therefrom are directed not to enter the through-holes.
It is preferable that the second wall portion has a configuration defined as a function of a horizontal distance between (a) a first boundary between the first and second recesses within the first wall and (b) an outer edge of the second recess, the horizontal distance being defined as a function of a thickness of the shadow mask, a height of the first boundary, a width of the through-hole, an incident angle of the electron beams at the first boundary, and an inner width of the first recess. For instance, the above-mentioned horizontal distance is defined by the following equation:
S3xe2x89xa7H2xc3x97tan xcex22
xcex22=(90xe2x88x92xcex1) /2
xcex1=tan (S2/H2)xe2x88x92
wherein: S3 indicates the horizontal distance; H2 indicates a height of the first boundary; xcex1 indicates an incident angle of the electron beams entering the through-holes; and S2 indicates a horizontal distance between (a) a second boundary between the first and second recesses within the second wall and (b) an outer edge of the second recess.
It is preferable that the second recess has a central axis located closer to a center of the shadow mask than a central axis of the first recess by a predetermined distance. The predetermined distance may be a function of a height of the first boundary, a thickness of the shadow mask, and an incident angle of the electron beam entering the shadow mask. It is preferable that the predetermined distance is set equal to or smaller than 50 xcexcm.
In another aspect of the present invention, there is provided a method of fabricating a shadow mask to be used for a cathode ray tube, including the steps of (a) forming a first photoresist pattern on a first surface of a shadow mask for forming a first recess at the first surface, (b) forming a second photoresist pattern on a second surface of the shadow mask for forming a second recess at the second surface in such a manner that the second recess cooperates with the first recess to thereby from a through-hole throughout a thickness of the shadow mask, that the second recess has a smaller size than that of the first recess, and that the second recess has a central axis located closer to a center of the shadow mask than a central axis of the first recess by a predetermined distance, (c) etching the shadow mask with the first and second photoresist patterns acting as a mask, and (d) removing the first and second photoresist patterns.
For instance, the predetermined distance is preferably set equal to or smaller than 20 xcexcm.
It is preferable in the step (c) that the shadow mask is etched so that a first boundary between the first and second recesses within a first wall is located lower than a second boundary between the first and second recesses within a second wall on the basis of a bottom of the second recess, the first wall being defined as a wall of the through-hole located farther away from a center of the shadow mask than the second wall. It is also preferable that the shadow mask is etched so that the first boundary has a height equal to or lower than 20 xcexcm on the basis of a bottom of the second recess. It is preferable that an etching pressure for forming the first recess is different from an etching pressure for forming the second recess.
In still another aspect of the present invention, there is provided a cathode ray tube including (a) a bulb having a face panel constituting a front surface of the bulb, and a neck portion, (b) a fluorescent film formed on an inner surface of the face panel, (c) an electron gun disposed in the neck portion of the bulb, (d) a deflecting yoke disposed around the neck portion of the bulb for deflecting electron beams emitted from the electron gun, and (e) the above-mentioned shadow mask disposed between the fluorescent film and the electron gun.
In accordance with the present invention, it is possible to direct electron beams reflected at the second wall portion in a direction different from a direction in which the electron beams are originally directed. For instance, the electron beams having been reflected at the second wall portion of the first wall are reflected towards an inner surface of the first recess or towards an electron gun. Accordingly, it is possible to prevent images from being unnecessarily formed on the fluorescent film, which ensures to avoid degradation in the contrast characteristic of the shadow mask.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.